The present invention relates to a transformerless wind turbine generator. In particular, the present invention relates to a wind turbine generator having a high-voltage power generator operatively connected to a high-voltage power converter so that the traditional step-up transformer can be omitted. Also, the present invention relates to high-voltage power rectifier/inverter/converter solutions suitable for use in high-voltage, transformerless wind turbine generators.
Traditionally, wind turbine generators are equipped with step-up transformers in order to match a 0.69-6 kVac generator voltage level with a grid voltage level of 10-35 kVac. However, there are several disadvantages associated with step-up transformers in wind turbine generators—the most significant of these being related to increased power conversion losses, increased component count, cost, weight and volume. Finally, step-up transformers induce an increased risk of turbine failures—potentially turbine breakdowns. Thus, there is a strong need and a wish to remove step-up transformers from wind turbine generators.
One immediate problem arising from a removal of a step-up transformer is that the power converter, typically including a rectifier and an inverter separated by a DC-link, must be operable at voltages levels of 10-35 kVac. Also, the power generator needs to be operable at such power levels. In order to operate power converters at a 10-35 kVac voltage level the individual valves of the rectifier and the inverter of the power converter must be capable of blocking such voltage levels. This may be achieved by connecting a number of, for example, IGBTs in the valves of the power converter in series.
However, connecting semiconductor devices like IGBTs in series is known to be a rather difficult task due to voltage balancing problems arising from different switching times of the IGBTs. If the voltage across series-connected IGBTs are not properly balanced overvoltage levels typically ends up on specific IGBTs. In particular, during turn off, the generated transient voltage is superimposed on the main voltage level increasing the risk of device damage. Thus, controlling the voltage balance is very critical when IGBTs are connected and operated in series.